Patent GB-1435555-A (GB)
Title: Electroacoustic transducers and diaphragms therefor
IPC: H04R7/04
Filed: 1973-07-23
Abstract: 1435555 Diaphragms; moving coil loudspeakers J W MANGER 23 July 1973 [25 July 1972] 34987/73 Heading H4J A diaphragm for an electro-acoustic transducer comprises a textile element impregnated with a filler. The diaphragm is elastically stretchable, flexible and so heavily damped as to be substantially incapable of converting mechanical strain energy therein into kinetic energy of the diaphragm. It may be circular or square and may be used in loud-speakers, microphones or headphones. Preferably it is of knitted textile, for example being formed of looped thread which allows continuousre-forming of the thread position during stretching. The thread may be of multifilament fibre. Nylon stretch yarns of S and/or Z twist may be used. The fibres may be elastic (e.g. polyamide or polyester) or inelastic (e.g. glass fibre). Fine lady's stocking material is satisfactory. The filler may be a butadiene copolymer. Preferably the weight ratio of textile to filler is 1 : 5. A loud-speaker (Fig. 1) has two pot magnets 8, 9 and two moving coils 12, 13 rigidly connected by a hollow cylinder, to the diaphragm 14. An electromagnetic arrangement, such as described in Specification 1,286,687 provides the equilibrium position. The periphery of diaphragm 14 is fixed to a stationary ring 19 while its inner portion is connected by a ring 18 to a coil former 16. An alternative arrangement (Fig. 2) has a single pot magnet 23 and the periphery of a circular diaphragm 26 is fixed to a stationary ring 27. A coil former 25 carrying two coils in tandem is also secured to the diaphragm 26. The centre of the diaphragm 26 may be held fixed by a pin fitted to the magnet system. Former 25 may divide the diaphragm 26 into two sections of equal area but preferably the diameter of former 25 is equal to the radius of diaphragm 26 or ring 27. In order to avoid an air chamber 28 there may be two annular pole shoes 29, 30 connected by a plurality of permanent magnets 31 arranged in a circle with air spaces 36 between the magnets 31. Alternatively, the air gap L (Fig. 3) has on one side two pole shoes 29a, 30a of permanent magnets 31 and, on the other side, a core 33 fixed to the pole shoes 29a, 30a by webs of non-magnetic material (not shown). A pin 35 supported by webs 34 holds the centre of diaphragm 26 stationary. At low frequencies the whole diaphragm 26 vibrates, all parts remaining in phase, but as frequency increases a progressively smaller part of the diaphragm 26 vibrates, from the former 25 outwardly. In manufacture, the coil former 25 (Fig. 2a) is centred with the air gap, one end being flush with the outer ring 27. A cut piece of stocking material is placed on former 25 and ring 27, which are horizontal. Weights are suspended from the edge of the stocking material, which is thereby stretched so that it does not form creases and the loops are straight and parallel. Leads from the moving coil may be drawn through the material. Diluted butadiene copolymer solution is applied to the material adjacent the former 25 and ring 27 to adhere these parts. The material is next coated on its front with butadiene copolymer solution, which is dried at 70 C. The back is then similarly treated. The coating is sufficiently thick so that no fabric end protrudes through it. A thicker layer is applied near the former 25 and ring 27. Subsequently the diaphragm is dried at 130 to 150 C. The diaphragm then relaxes and forms creases which allow slight axial movement (e.g. 10 to 20 mm.) without radial stetching. Alternatively, after weighting the edge, the material is clamped to the ring 27 and the former 25 is arranged concentrically using an auxiliary fitting. The arrangement is passed under a curtain of butadiene copolymer solution.